1. Field of the Invention
This invention relates to an air-fuel ratio-control system for an internal combustion engine, which has air-fuel ratio sensors provided at respective locations upstream and downstream of a catalytic converter arranged in the exhaust system of the engine, and controls the air-fuel ratio of an air-fuel mixture supplied to the engine, based on outputs from the air-fuel ratio sensors, and more particularly to an air-fuel ratio control system of this kind, which has a function of detecting deterioration of the catalytic converter.
2. Prior Art
There is conventionally known an air-fuel ratio control system for internal combustion engines, which has, as air-fuel ratio sensors, oxygen concentration sensors (hereinafter referred to as "the O2 sensors") arranged, respectively, upstream and downstream of a three-way catalyst as a catalytic converter arranged in the exhaust system of the engine, and carries out feedback control of the air-fuel ratio of a mixture supplied to the engine (hereinafter referred to as "the supplied air-fuel ratio"), based on outputs from the O2 sensors.
To detect the deterioration degree of the catalytic converter, an air-fuel ratio control system of this kind has been proposed by the present assignee, as disclosed in U.S. Ser. No. 07/961,794 filed Oct. 16, 1992, according to which, during execution of feedback control in which an air-fuel ratio correction coefficient (air-fuel ratio correction amount) KO2 is calculated based only on an output from the downstream O2 sensor, measurements are made of a time period TL elapsed from a time point a special P term (proportional term) for skipping the value of the correction coefficient KO2 in a decreasing direction is generated to a time point the output from the downstream O2 sensor is inverted correspondingly, and a time period TR elapsed from a time point a special P term for skipping the KO2 value in an increasing direction to a time point the output from the downstream O2 sensor is inverted correspondingly, and then an average value of these measured time periods TL and TR are calculated, to thereby determine that the catalyst is deteriorated when the calculated average value is smaller than a predetermined value, based on the fact that the average value decreases with advancement of deterioration of the performance of the catalyst (O2 storage capacity).
Further, in the above proposed control system, similarly to the conventional air-fuel ratio control system in general, predetermined upper and lower limit values are provided for the KO2 value in order to detect failure of a fuel injection valve, such as fuel leakage and clogging of the valve element or the fuel supply pipe. That is, if fuel leaks from the fuel injection valve into the intake pipe, an amount of fuel supplied into the cylinder becomes larger than a desired fuel amount. As a result, the KO2 value decreases to be held at the lower limit value. On the other hand, clogging of the fuel injection valve prevents fuel from being supplied to the cylinder, so that the KO2 value increases to be held at the upper limit value.
In the above proposed air-fuel ratio control system, in addition to failure of a fuel injection valve or valves, the KO2 value decreases to be held at the lower limit value when a large amount of evaporative fuel is purged from a canister to the intake pipe.
When the KO2 value is held at the lower limit value during detection of deterioration of the catalytic converter, the actual fuel injection amount becomes larger than the desired amount, so that the output from the downstream O2 sensor continues to assume a value indicative of a rich air-fuel ratio without inversion. On the other hand, when the KO2 value is held at the upper limit value, the actual fuel injection amount becomes smaller than the desired amount, so that the output from the downstream O2 sensor continues to assume a value indicative of a lean air-fuel ratio without inversion.
When the KO2 value is thus held at the upper limit value or the lower limit value, the feedback control based only on the output from the downstream O2 sensor becomes unstable, and the calculated average values of the time periods TL and TR do not properly correspond to the storage capacity of the catalytic converter. As a result, there is a fear that the determination of deterioration of the catalyst cannot be accurately performed.